I like to buy Speedball Inks. They're relativly cheap, widely available, and come in primary color sets.
Take note that the ink on the right says fabric ink and the ink on the left just says acrylic. The fabric inks are more permanent on cotton and can be more difficult to clean up. They're more expensive too.
For this demo I'm printing on a t-shirt so I'll be using my fabric ink.
Do not buy oil based inks or plastisol inks unless you know what you're doing. They're mega-toxic and not what you want for DIY.